Flavor-changing neutral current

In theoretical physics, flavor-changing neutral currents (FCNCs) are hypothetical expressions that change the flavor of a fermion current without altering its electric charge. If they occur in nature (as reflected by Lagrangian interaction terms), these processes may induce phenomena that have not yet been observed in experiment. Flavor-changing neutral currents may occur in the Standard Model beyond the tree level, but they are highly suppressed by the GIM mechanism. Several collaborations have searched for FCNC. The Tevatron CDF experiment first observed the FCNC decay of the strange B-meson to phi mesons in 2005.

FCNCs are generically predicted by theories that attempt to go beyond the Standard Model, such as the models of supersymmetry or technicolor. Their suppression is necessary for an agreement with observations, making FCNCs important in model-building.

Consider a toy model in which an undiscovered boson S may couple both to the electron as well as the tau via the term

Since the electron and the tau have equal charges, the electric charge of S clearly must vanish to respect the conservation of electric charge. A Feynman diagram with S as the intermediate particle is able to convert a tau into an electron (plus some neutral decay products of the S). The MEG experiment at the Paul Scherrer Institute near Zurich will search for a similar process, in which an antimuon decays to a photon and an antielectron. In the Standard Model, such a process proceeds only by emission and re-absorption of a charged W boson, which changes the tau into a neutrino and then an electron, emitting a photon to conserve energy and momentum.

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